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     The Chehalis River Basin in western Washington is the second largest basin by area in Washington State, second only to the Columbia River Basin.  Originating in the Willapa Hills, the Chehalis River travels approximately 125 miles north-northwest through the Chehalis River basin before discharging into the Pacific Ocean through Grays Harbor Estuary.  The basin is located within the Grays Harbor, Lewis, Thurston, Cowlitz, Jefferson, Mason, Pacific, and Wahkiakum Counties.  The Chehalis River basin drains 2,660 square miles while the Chehalis River covers approximately 3,350 linear stream miles.  This basin contains 180 lakes, ponds, and reservoirs providing water for agriculture, fish habitat, and wildlife.

     The Chehalis River basin has a unique geography that includes mountain ranges, flat and open river valleys, hills and sloping terrains, and close proximity to the Pacific Ocean.  The basin is bordered by the Olympic Mountain in the north, the Deschutes River basin in the east, the Willapa Hills and Cowlitz River basin in the south, and the Pacific Ocean on its west side.  Elevations range from 100 feet to 2,700 feet at the tallest mountain in the Black Hills, Larch Mountain.  On the fringes of the floodplains, there is cropland, pasture areas, and the dominant flora: western red cedar, red alder, black cottonwood, and willow species.  Well-drained soil areas also contain Douglas-fir trees.  Forested land constitutes 77 percent of the Upper Chehalis Basin and 91 percent of the lower basin. Most of these forested lands are private and government-owned property.  The west side of the Chehalis River primarily supports agriculture, which comprises approximately 257 square miles, or 9.7 percent of the basin area.  The east side of the river has developed into the Centralia and Chehalis urban centers.

     Hydrology is the “science dealing with the properties, distribution, and circulation of water on and below the earth’s surface and in the atmosphere. “ (Miriam-Webster Dictionary, 2011).  The Chehalis River basin has been shaped by a unique geologic history with primary geologic units including volcanic and sedimentary rock and alluvial material.  This complex geologic history has shaped much of the distribution, quantity, and movement of waters through the Chehalis River basin.

     The Chehalis River originates in the Coast range, in the Willapa Hills.  It then flows east to Chehalis, north to Grand Mound, and finally west to Gray’s Harbor.  The main tributary rivers upstream of the Chehalis River are the Newaukum, Skookumchuck, Black, Satsop, Wishkah, and Wynoochee Rivers.   Creeks that drain into the Chehalis River include the Salzer, Dillenbaugh, China, Scatter, Porter, and Cloquallum Creeks.

     The groundwater in the Chehalis River basin create surficial and alluvial aquifers.  Surficial aquifers are water systems that occur between several feet below ground surface to depths of 100 feet.  The primary surficial aquifers in the basin are located in the river valleys and upland prairies.  The wells associated with these water sources can generate between 200 and 3,000 gallons per minute.  Alluvial aquifers are much shallower than the surficial aquifers.  These occur within 20 feet of the ground surface, and provide water for local farms, private residences, and public water systems. 

     Surface waters in the Chehalis River basin are primarily supported by rainfall precipitation, although mountains in the area accumulate snow during the winter season.  Snowmelt primarily affects the Satsop and Wynoochee Rivers.  The Wynoochee Dam on the Wynoochee River, is the only structure in the basin authorized to serve the purpose of flood control.  

     The Chehalis River basin also contains estuarine and tidal wetlands coupled with forested, scrub-shrub, emergent, and riverine wetlands. Wetlands are those areas inundated or saturated by
surface water or groundwater at a frequency and duration to support vegetation typically
adapted for life in saturated soil conditions (RCW 36.070A.030).  Wetlands naturally help control flood symptoms because they serve retention and detention functions in times of increased drainage. The variety of wetlands in the Chehalis River basin range from temporarily flooded to seasonally and permanently flooded. 

     The 2000 U.S. Census estimated the Chehalis River basin population at 141,000 people (see Table 1.  Population of Basin Jurisdiction, below).  In addition to the Confederated Tribes of the Chehalis Reservation located in the basin, the basin contains four population centers: Chehalis, Centralia, Aberdeen, and Hoquiam.   These centers contain much of the basin’s industrial development, including the coal mine/power plant site south of Bucoda.  These areas depend on surface waters in the basin for their municipal and industrial supplies. The principal industrial use of water is for the manufacturing of wood, pulp, and paper products. See Figure 1. Land Use and Land Cover in the Chehalis River Basin, below, for visual reference about developed and forested areas in the basin.

Figure 1. Land Use and Land Cover in the Chehalis River Basin

^{Image by Earth Economics Report for the Chehalis River Basin Flood Authority, 2010. Showing data pixels utilized in ARIES modeling.}

Agriculture makes up approximately seven percent of land use in the Chehalis River basin.  Intensive agriculture and irrigation plans are generally located in the lower basin where there are low-lying valleys adjacent to the Chehalis River and its major tributaries.   In 1975, the U.S. Department of Agriculture estimated that agriculture in the basin primarily consisted of crop production (133,000 acres), with pasture comprising 1.8 percent (31,000 acres) of the basin.   The most common crops include hay and silage, vegetables and small grains. Pasture includes livestock and dairy production.

Major transportation routes run parallel to the Chehalis River.  These include State Route 6 from Pe Ell to Chehalis, Interstate 5 from Chehalis to north of Centralia Highway 12 from Interstate 5 to near Elma, and U.S. Highway 101 from Elma to the river mouth.

^{Source: Office of Financial Management, 2010}